Somatomedins (also sometimes referred to as "SMs") are hormones having useful biological properties. SMs are polypeptides having a molecular weight of approximately 7,500 daltons. SMs (a) mediate the growth-promoting effects of growth hormone. (also sometimes referred to as "GH"), (b) have weak insulin-like activity (and for that reason are also called "insulin-like growth factors" or "IGFs"), (c) are mitogenic for a variety of skeletal and other tissues and (d) are transported in plasma bound to a large carrier protein. There are two SM compositions in humans. SM-C is a basic polypeptide and is sometimes referred to as SM-C. SM-C mediates the growth promoting actions of GH after birth. SM-A is a mixture primarily of a polypeptide known as IGF-II and variable amounts of a modified form of SM-C. Spencer, E. M., et al., "The Identity Of Human Insulin-like Growth Factors I and II With Somatomedins C and A With Rat SM I and II" in Insulin-like Growth Factors/Somatomedins; ed. Spencer, E. M. (Walter de Gruyter 1983). IGF-II is less GH dependent and may have a role in fetal growth.
SMs may be useful in vivo to stimulate bone formation (for example, in treatment of osteoporosis), wound healing, and the growth of animals and GH-deficient humans. Serum levels of SM-C are measured to diagnose acromegaly, pituitary gigantism, GH deficiency, and other growth related conditions. Spencer, E. M., "Somatomedins" in Basic and Clinical Endocrinology, ads. Greenspan F. S. and Forsham, P. H. (1986), p. 89, Appleton-Century-Crofts. SMs are also employed to stimulate in vitro the proliferation of a variety of cells in tissue culture and, therefore, are useful in the study of the regulation of normal and abnormal cell growth. SMs produced by certain breast and kidney cancer cells may stimulate the proliferation of both the cancer cells and the vascular and fibrous tissues required to support the growth of the cancer tissues. Spencer, E. M. et al., "Possible Auto-stimulation of Human Mammary Carcinoma Growth by Somatomedins," Annals of the New York Acad. Sci., 464, p. 448 (1986): Huff, K. K., et al., "Secretion of Insulin-like Growth Factor-I-related Protein by Human Breast Cancer Cells," Cancer Research 46, pp. 4613-4619 (1986). Blocking the action of SMs may be useful to control the growth of these cancers.
Human SMs appear to be transported and regulated in vivo by other proteins. Hintz, R. L. et al., "Demonstration of Specific Plasma Protein Binding Sites For Somatomedin," J. Clin. Endocrinol. Metab. 45, p. 988 (1977). These proteins appear to bind to the SMs and regulate the biological activity of the SMs in vivo. Gel filtration of human serum at neutral pH has shown that 95% of the immunoreactive SM-C activity, and probably IGF-II activity, elutes at about 150,000 to 160,000 daltons (150-160 kilodaltons or "kDa") with a minor amount in the range of 35-50 kDa. Only a very small amount of immunoreactive activity elutes at 7.5 kDa, where free SMs should appear. Smith, G. L., Molecular and Cellular Endocrinology 34, p. 83-89 (1984). This indicates that SMs are complexed with larger proteins in plasma.
At least two different classes of proteins or protein complexes in human plasma have been reported to bind SMs. Drop, S. L. et al., "Immunoassay Of A Somatomedin-binding Protein From Human Amniotic Fluid; Levels In Fetal, Neonatal, And Adult Sere," J. Clin. Endocrinol. Metab. 59, p. 908 (1984): Wilkins, J. R. et al., "Affinity-labeled Plasma Somatomedin-C+/Insulin-like Growth Factor I Binding Proteins," J. Clin. Invest. 75, p. 1350 (1985). This description refers to one class of those native proteins or protein complexes as the SM "Carrier Protein"for its function appears to be the transport of SMs. This term is not intended to indicate that the carrier protein is a single protein. There may be more than one carrier protein and it may be a protein complex. This description refers to the other class as the "Amniotic Fluid Binding Protein" or "AFBP." There may be more than one AFBP. It is also possible that additional classes of proteins or protein complexes that bind SMs will be discovered.
Carrier protein activity, like SM-C activity, is GH-dependent, being low in persons with GH deficiency and elevated in patients with GH-producing tumors, a condition known as acromegaly. White. R. M., et al., "The Growth Hormone Dependence Of Somatomedin-binding Protein In Human Serum." J. Clin Endocrinol Metab. 53, p. 49 (1981). The carrier protein displays biological properties indicative of potentially valuable uses. In vivo, when SMs bind to carrier protein, the half-life of the SMs is reported to increase from approximately one hour to up to about 24 hours depending on the animal species tested (Cohen, K. L. et al., "The Serum Half-life Of Somatomedin Activity: Evidence For Growth Hormone Dependence," Acta Endocrinol. 83, p. 243 (1976)), and the SMs are rendered inactive until released. Studies in other model systems suggest that impure preparations containing the carrier protein (a) abolish the metabolic action of the SMs on the perfused rat heart (Meuli C., et al., "NSILA-carrier Protein Abolishes The Action of Nonsuppressible Insulin-like Activity (NSlLA-s) On Perfused Rat Heart," Diabetologia 14, p. 255 (1978)), (b) inhibit the mitogenic effect of the SMs on cells in culture (Knauer, D. J., Proc. Natl. Acad. Sci. U.S.A., 77, pp. 7252-7256 (1980) and Kuffer, A. D., et al., "Partial Purification Of A Specific Inhibitor Of The Insulin-like Growth Factors By Reversed Phase High-performance Liquid Chromatography," J. of Chromatography, 336, pp. 87-92 (1984) and (c) block the insulin-like activity of SMs on rat adipose tissue (Zapf, J., et al., "Inhibition Of The Action Of Nonsuppressible Insulin-like Activity On Isolated Rat Cells By Binding To Its Carrier Protein," J. Clin Invest. 63, p. 1077 (1979). Partially pure preparations of the carrier protein have been used with radiolabeled SMs in research to conduct competitive binding assays for measuring SMs. Moses, A. C., et al., Endocrinology 104, p. 536 (1979).
Because of their valuable biological properties, there have been many efforts to isolate and characterize the carrier protein or the subunits of the carrier protein responsible for that activity. Prior to this invention, all attempts to isolate and characterize in pure for the carrier protein or its active subunits have failed. This is due in part to the low concentration of carrier protein in plasma. A successful purification procedure also had to solve the problems of loss of activity because of enzymatic digestion and instability of the carrier protein, especially to changes in pH: Purification of the carrier protein subunits is further complicated by the presence in plasma of the AFBP, which also binds to somatomedins.
The carrier protein is a glycoprotein. In serum at neutral pH, it is bound with SMs and the complex has a molecular weight of about 150-160 kDa when measured by gel filtration. The molecular weight of the carrier protein complex at neutral pH has also been determined by other methods to be about 125 kDa. Gel filtration chromatography of serum or plasma under acid conditions has been reported to separate bound SMs from the carrier protein and to give rise to a unit of the carrier protein that has a molecular weight of about 40-50 kDa. That unit also binds to somatomedins. Hintz, R. L., et al., "Demonstration Of Specific Plasma Protein Binding Sites For Somatomedin," J. Clin. Endocrinol. Metab. 45, p. 988 (1977). Since the 40-50 kDa acid-stable unit cannot be induced to reform the 150-160 kDa carrier protein complex, others have suggested that the carrier protein may also be composed in part of an acid-labile unit that does not itself bind to somatomedins. Moses, A. C., et al., Endocrinology 104, p. 536 (1979). Furlanetto reported treating serum with a 35-55% ammonium sulfate solution, isolating the precipitate, dissolving the precipitate in 0.05M Tris. pH 8.20 and chromatographing on DEAE Sephadex A-50 with Tris buffers. Furlanetto, R. W., "The Somatomedin C Binding Protein: Evidence For A Heterologous Subunit Structure," J. Clin. Endocrinol Metab. 51, p. 12 (1980). Furlanetto did not disclose any further purification. Rather, Furlanetto conducted experiments with various fractions to confirm his view that the somatomedin-C binding activity in serum is composed of at least two units, one has a Stokes' radius of 36 A.degree. and binds SM-C (the so-called acid stable unit) and the other has Stokes' radius of 30-38 A.degree. and does not bind-SM-C (the so-called acid labile unit)).
Wilkins identified, by affinity labeling, plasma proteins that complexed with SM-C. Wilkins, J. R., et al., "Affinity-labeled Plasma Somatomedin-C/Insulin-like Growth Factor I Binding Proteins," J. Clin. Invest., 75, p. 1350 (1985). .sup.125 I-SM-C was covalently cross-linked to proteins that bound SM-C in whole plasma and in Sephadex G-200 fractions of plasma. Following sodium dodecylsulfate polyacrylamide gel electrophoresis and autoradiography, the AFBP was identified in addition to species of about 160, 110, 80, 50 and 25 kDa. Wilkins et al. hypothesized that the 160 kDa carrier protein complex consisted of 6 approximately 25 kDa (24-28 kDa) subunit complexes, each composed of the subunit plus SM-C. However, Wilkins et al., did not report isolation or purification of this 25 kDa subunit. Another worker proposed, but did not establish, a slightly larger subunit structure. Daughaday, W. H., et al., "Characterization Of Somatomedin Binding in Human Serum By Ultracentrifugation And Gel Filtration," J. Clin. Endocrinol. Metab. 55, p. 916 (1982).
Several workers have reported unsuccessful attempts to isolate the acid-stable 40-50 kDa carrier protein unit from human plasma. Draznin et al., reported a material containing only 1% SM binding activity and did not disclose whether this material originated from carrier protein or AFBP. Draznin. B., et al., in "Somatomedins and Growth." eds. G. Giordano et al. (Academic Press 1979) pp. 149-160. Fryklund et al., fractionated fresh frozen human plasma by polyethylene glycol precipitation, carboxymethyl-Sephadex chromatography, and gel filtration. Fryklund, L., et al., in Hormones and Cell Culture, eds G. H. Sato et al. (Cold Spring Harbor Laboratory 1979) pp. 49-59. Fryklund et al., proposed that the carrier protein consisted of 2 dissimilar chains of 35 and 45 kDa. Fryklund et al., disclosed that glycine was released by N-terminal molecule analysis, but did not identify from which chain it originated or whether both ended in glycine. The reported binding activity of the Fryklund et al. preparation was very low and purity was not reported. Fryklund et al. did not establish whether the carrier protein or the AFBP was present in their preparation. Morris et al., reported obtaining crude SM binding fractions by acetic acid extraction of human Cohn fraction IV, incubation with I-IGF-I and chromatography on Sephacryl S-200. Morris, D. H., et al., "Structure of Somatomedin-binding Protein: Alkaline pH-Induced Dissociation of an Acid-Stable, 60,000 Molecular Weight Complex Into Smaller Components," Endocrinology 111, pp. 801-805 (1982). Morris et al. described fractions containing bound radioactive SM-C with apparent molecular weights of 60,000 and 46,000. Morris et al. reported that exposing these fractions to pH 8.0 resulted in a shift of .sup.125 I-IGF-I binding activity from 60,000 and 46,000 daltons to fractions with complexes of 46,000 and 30,000. These fractions were not further purified. Martin et al. reported preparing a polyclonal antibody to the acid-stable unit. The latter was isolated by extracting human Cohn fraction IV with 2M acetic acid, 75 mM NaCl. After removal of SMs by adsorption to SP-Sephadex, the acid stable unit was obtained by IGF-II-Affinity Chromatography and used for immunization. Martin et al. disclosed that HPLC could further purify the acid stable unit. No data was supplied to establish the purity of their final product. Martin, J. L., et al. "Antibody Against Acid-Stable Insulin-Like Growth Factor Binding Protein Detects 150,000 Molecular Weight Hormone-Dependent Complex In Human Plasma." J. Clin. Endocrinol. Metab. 261, pp. 799-801 (1985). Kuffer et al. reported a partial purification of what he described as an inhibitor of insulin-like growth factors (SMs). Kuffer. A. D. et al., "Partial Purification of A Specific Inhibitor of the Insulin-Like Growth Factors By Reverse Phase High-Performance Liquid Chromatography," J. of Chromatography, 336, pp. 87-92 (1984). Kuffer et al. prepared SM inhibitors having a molecular weight of 16,000 to 18,000 from Cohn fraction IV-1 by ion exchange chromatography and sequential gel chromatography under acid conditions on Sephadex G-75 and Bio-Gel P-30 columns. After affinity chromatography and high performance liquid chromatography, Kuffer et al. obtained the "inhibitory activity" as two peaks of activity, corresponding "to a major, apparently homogeneous, protein peak and a minor heterologous peak." Kuffer et al. did not report isolation of the activity of either peak.
None of the above studies disclose a class of human carrier protein subunits capable of binding somatomedin-like polypeptides. In addition, none of these studies disclose any subunits of the carrier protein capable of binding SMs and purified to homogeneity. Purity is required to establish that the carrier protein has been isolated instead of the AFBP or a contaminant and to study biologic activity. An impure preparation may contain enzymes, causing the product to be unstable, and easily degraded or denatured. Impure preparations also cannot be used in animals and humans, because many impurities present in original serum or produced as a result of the purification procedures, are antigenic and could produce unwanted biologic effects. For example, human use in osteoporosis requires removal of all contaminants, which may be antigenic or have adverse biologic effects.
Other workers have isolated a different protein capable of binding SMs and obtained from mid-gestational amniotic fluid of humans, the amniotic fluid binding protein or "AFBP." The AFBP is not the carrier protein or a subunit of the carrier protein. Wilkins. J. R. et al., "Affinity-labeled Plasma Somatomedin-C/Insulin-like Growth Factor I Binding Proteins." J. Clin. Invest. 75, p. 1350 (1985). The AFBP (a) is smaller than the so-called acid-stable unit of the carrier protein, having a molecular weight in the range 32-40 kDa, (b) is not glycosylated, (c) differs from the carrier protein subunits of this invention in its reported N-terminal molecule (Povoa, G. et al., "Isolation And Characterization of A Somatomedin-binding Protein From Mid-term Human Amniotic Fluid," Eur. J. Biochem. 144, pp. 199-204 (1984)), and (d) has different immunologic properties. Drop, S. L. S. et al., "Immunoassay of A Somatomedin-Binding Protein From Human Amniotic Fluid: Levels In Fetal, Neonatal and Adult Sere," J. Clin. Endocrinol. Metab. 59, p. 908 (1984); Martin, J. L. et al., Supra, J. Clin. Endocrinol. Metab. 61, pp. 799-801 (1985). Antisera to the AFBP do not cross-react with the 150 kDa carrier protein or its acid-stable unit. Drop et al. reported that the AFBP levels determined by radioimmunoassay (RIA) were found to decrease during infancy and childhood--the inverse of the carrier protein and also, unlike the carrier protein, to have a significant diurnal variation. Enberg also isolated the AFBP from adult human plasma by four chromatographic steps: CM-Affigel blue, hydroxylapatite, fast protein liquid chromatography gel permeation and high performance liquid chromatography ("HPLC") hydroxylapatite. Enberg, G., "Purification of A High Molecular Weight Somatomedin Binding Protein From Human Plasma," Biochem, and Biophy. Res. Commun., 135, pp. 178-82 (1986). Enberg reported a "possible" N-terminal molecule, Ala-Pro-Trp-, demonstrating that the AFBP was isolated, not the 150 kDa carrier protein as Enberg erroneously concluded.
Proteins that bind SMs have also been identified in cell culture extracts (e.g., Adams, S. O., et al. Endocrinology 115, pp. 520-526 (1984)). Thus far, the carrier protein has not been isolated. Spencer first showed that primary cultures of liver cells produced a protein that complexes with SMs. Spencer, E. M, "The Use Of Cultured Rat Hepatocytes To Study The Synthesis Of Somatomedin And Its Binding Protein," FEBS Letters, 99, p. 157, (1979). Subsequently, several cell types, normal and abnormal, have been found to synthesize a protein that complexes with SMs. Cultured Buffalo rat liver tumor cells (BRL 3A) produce a 33 kDa SM binding protein that differs from the carrier protein by antibody reactivity, N-terminal amino acid molecule, and absence of glycosylation. Lyons R. M. et al., Characterization of Multiplication-Stimulatory Activity "MSA" Carrier Protein, "Molecular and Cellular Endocrinol. 45, pp. 263-70 (1986). Mottola. C. et al., J. of Biol. Chem., 261, pp. 1180-88 (1986). Romanus et al. reported that antibodies to this binding protein cross-reacted with a protein present in fetal serum but not adult rat serum. Romanus, J. A. et al., "Insulin-like Growth Factor Carrier Proteins In Neonatal And Adult Rat Serum Are Immunologically Different: Demonstration Using A New Radioimmunoassay For The Carrier Protein From BRL-3A Rat Liver Cells," Endocrinology, 118, p. 1743 (1986). The BRL-3A binding protein may be the rodent equivalent of the AFBP, but the N-terminal molecule data show no similarity between the two molecules.
Many proteins and polypeptides have been produced by use of recombinant DNA techniques. There is no published report of production of carrier protein-like polypeptides in this manner. There are numerous obstacles to using the techniques of recombinant DNA technology to clone and express a carrier protein-like polypeptide gene. Obtaining a gene encoding a carrier protein-like polypeptide is difficult for a variety of reasons. Prior to the invention, the protein sequences of the carrier protein and the carrier protein subunits were unknown and, therefore. DNA molecules that, would code for the subunits were unknown. No human tissue source was established. Fibroblasts had been shown to produce small amounts of a large uncharacterized SM binding protein (Adams. S. O., et al. Endocrinology 115, pp. 520-526 (1984)). While liver is the major source of somatomedins, it had never been shown to produce the carrier protein. In addition, the liver is difficult to use to isolate mRNA, due to ribonucleases, the quantities of carrier protein in serum are very low. Thus, mRNA might be rare. The genome including a DNA molecule coding for the carrier protein may contain intervening sequences. For these and other reasons, many pitfalls faced the conventional approach to attempt to isolate a gene encoding a carrier protein-like polypeptide --namely, identifying a source of mRNA containing large amounts of the desired molecule, creating a library of cDNA from that mRNA, screening the library with oligonucleotide probes designed to hybridize with cDNA having the desired molecule, and isolating or assembling a gene from those cDNA molecules.